See the WPCF

Inside the Waste Facility
This is the disinfection area. As you can see the ultraviolet lamps used for disinfection are under the floor, in the water. Ultraviolet light is a form of light that is invisible to the human eye. UV light is an environmentally-friendly, chemical-free, highly effective way to safeguard water against harmful microorganisms. The microorganisms in the water are exposed to UV light when they pass by special lamps submerged in the wastewater. The UV energy instantly destroys the genetic material (DNA) within bacteria, viruses and protozoa, eliminating their ability to reproduce and cause infection. Unable to multiply, the microorganisms die and no longer pose a health risk.